Some embodiments relate to a method for the preparation of defect-free nanosized synthetic zeolite materials, to the defect-free nanosized synthetic zeolite materials, to stable colloidal suspensions of the defect-free synthetic zeolite materials, and to the use of the defect-free nanosized synthetic zeolite materials and the stable colloidal suspensions in various applications.
Zeolites and zeolite-like materials include a broad range of porous crystalline solids. The structures of zeolite-type materials are essentially based on tetrahedral networks which encompass channels and cavities. According to ©2001 IUPAC [Pure Appl. Chem., 2001, 73, 2, 381-394], microporous crystalline materials with an inorganic, three-dimensional host structure composed of fully linked, corner-sharing tetrahedra and the same host topology constitute a zeolite framework type. The number of established framework or structure types has increased progressively in the last 4 to 5 decades of highly active research in the field of zeolites. Currently, the number of established structure types is clearly in excess of 229. All zeolite structure types are referenced with three capital letter codes. They have different framework densities, chemical compositions, dimensional channel systems and thus, different properties.
Zeolites are generally characterized by their high specific surface areas, high micropore volume, and capacity to undergo cation exchange. Therefore, they can be used in various applications, for example as catalysts (heterogeneous catalysis), absorbents, ion-exchangers, and membranes, in many chemical and petrochemical processes (e.g. in oil refining, fine- and petro-chemistry).
Most of the described zeolites are aluminosilicate zeolites and basically include a three-dimensional framework of SiO4 and AlO4 tetrahedra. The electroneutrality of each tetrahedra containing aluminum is balanced by the inclusion in the crystal of a metallic cation, for example a sodium cation. The micropore spaces (channels and cavities) are occupied by water molecules prior to dehydration. Pure-silica zeolites (e.g. pure-silica zeolite BEA, pure-silica zeolite MFI and pure-silica zeolite MEL), which have only silicon and virtually no aluminum, were also extensively studied because of their high hydrophobic character which imparts improved zeolite catalytic stability in aqueous environments, and improved catalytic activity and selectivity in reactions where molecules of different polar character are involved.
In parallel, the synthesis of zeolites in the presence of other metallic elements such as Ti, Fe, Co, Cu, Sn, Zr, Nb is an important research area in molecular sieve science since the introduction of metallic elements with different oxidation states and electronegativity modifies inter-alia their acidic and ion-exchange properties. As an example, WO97/33830 A1 describes the preparation of zeolite Ti-beta without aluminum, in the presence of hydrofluoric acid (HF) and in the absence of seed, by hydrolyzing a mixture of tetraethyl orthosilicate, tetraethylammonium hydroxide, water and hydrogen peroxide (H202); adding tetraethyl orthotitanate to the resulting solution and pursuing hydrolysis; evaporating off the ethanol; adding HF to the resulting mixture; and heating in an autoclave at 140° C. during 11 days. Such zeolite preparation is long and the fluoride anions coming from harmful HF lead to crystallization of big crystals. In addition, traces of fluoride anions are still present in the final crystalline zeolite product. In addition to the above-mentioned heteroatoms, transition metal ions (e.g. W, Mo, V, Cr ions) loaded onto several zeolites (e.g. HZSM-5, HZSM-11, HZSM-8, H-beta, HMCM-41, HMCM-49, HMCM-22, HY and H-mordenite) have been widely studied as catalysts for selective methane conversion reaction into benzene (also called dehydroaromatization of methane or DHAM). These catalysts are generally prepared by impregnation of the zeolite in an aqueous solution containing a salt of the transition metal ion (e.g. ammonium metatungstate) in acidic medium. The transition metal ions species are mainly located on the zeolite external surface and some of them diffuse into the internal channels of the zeolite. However, they are not an integral part of the zeolite framework.
Over the past decade, renewed efforts were devoted to prepare zeolites with enhanced accessibility to their micropores, including post-synthesis modification, one-step hydrothermal crystallization in the presence of mesopore modifiers and synthesis of nanosized zeolite crystals with or without organic templates. The interest in the preparation of nanosized zeolites has gradually increased, but only 18 from the 229 structures known to date have so far been synthesized with nanosized dimensions and stabilized in colloidal suspensions. Indeed, the particle size reduction of zeolites to the nanometer scale leads to substantial changes in their properties such as increased external surface area and decreased diffusion path lengths (which can lead to pore blocking by coke formation). More particularly, the specific conditions employed to lead to nanosized zeolites change their intrinsic characteristics, impeding the full use of their potential. Thus, all nanosized zeolites invariably contain significant levels of framework and surface defects. By way of example, pure-silica nanosized zeolites such as nanosized Silicalite-1 (MFI-type structure) and nanosized Silicalite-2 (MEL-type structure) still exhibit hydrophilic properties related to water adsorption on defect sites. Indeed, zeolites nanocrystals are generally synthesized at high pH values in alkaline or OH− medium (i.e. in the presence of hydroxide ions) and in the presence of a structure directing agent (i.e. SDA). Under such conditions, a large number of silanol nests is formed and the silanol groups obtained condense at high temperature (e.g. during calcination) to form Si—O—Si bridges. However, condensation is generally not complete and calcined zeolites still contain framework defects. Framework defect sites are prone to retain coke precursors, resulting in lower catalytic activity and faster deactivation.
The synthesis of zeolites in fluoride media (i.e. in the presence of fluoride ions) was developed to prepare defect-free zeolites (i.e. zeolites without framework defects), which improves their hydrophobic character and thus their catalytic activity. As an example, Camblor et al. [J. Mater. Chem., 1998, 8, 9, 2137-2145] described the preparation of zeolite beta by contacting a source of silicon (e.g. tetraethylorthosilicate), a SDA (e.g. tetraethylammonium hydroxide reagent TEAOH) and a source of aluminum (e.g. metal aluminum), and then adding, fluoride anions F− (coming from HF). However, the zeolite crystals harvested in fluoride media are considerably larger (e.g. 0.5-3 μm) than those synthesized in alkaline medium because of their lower nucleation rate. Moreover, the obtained crystallites are agglomerated. Thus, their applications in various fields are hampered by severe diffusion limitations. In addition, a fluoride medium (HF in particular) raises serious safety issues for a large-scale implementation.
In order to improve the synthesis of nanosized crystals in fluoride medium, Qin et al. [Adv. Funct. Mater., 2014, 24, 257-264] described a seeded fluoride medium synthesis of nanosized zeolites ZSM-5 by contacting a source of silicon, a source of aluminum, fluoride anions F− (coming from ammonium fluoride reagent NH4F), a SDA (e.g. tetra-n-propylammonium hydroxide reagent TPAOH); and adding silicalite-1 seeds. However, the crystal size is not optimized (e.g. nanocrystals of at least 150 nm) and silanol nests are still present.
Burel et al. [Microporous and Mesoporous Materials, 2013, 174, 90-99] recently reported the preparation of nanosized pure silica zeolite crystals by post-synthesis fluoride treatment. More particularly, nanosized silicalite-1 crystals were prepared according to the conventional hydroxide route and then treated with NH4F under relatively mild conditions. The density of framework connectivity defects is decreased. However, the zeolite hydrophobicity and the size of the zeolite crystals are still not optimized (e.g. nanocrystals of about 150 nm).